Heating of liquids and gases (e.g., fluids) can be carried out by a variety of techniques including conduction, radiation and convection and this heat may be transferred to other objects in thermal communication with the heated fluid. A wide variety of thermal processing applications for heating are found throughout industry including materials processing and chemical applications. The industrial process of heat-treating, joining, curing and drying are carried out in many different types of systems, furnaces and ovens. The heating method of choice for such applications is normally a radiative technique with radiant electric heating elements placed along the walls of the furnace. Although such a method is efficient for very high temperature applications, the use of convection as the heat transfer mechanism often proves to be efficient in the lower temperature ranges. The following prior art patents all pertain to various methods of heating gases; namely, U.S. Pat. Nos. 5,766,458; 5,655,212 and 5,963,709. Discussions on convective heating are available from (1) M. Fu, Kandy Staples and Vijay Sarvepalli. A High Capacity Melt Furnace for Reduced Energy Consumption and Enhanced Performance. Journal of Metals (JOM), May 1998, pg 42 and (2) ADVANCE MATERIALS & PROCESSES magazine (pages 213 to 215, October, 1999).
The proper selection of thermal heating for industrial applications such as processing ovens and furnaces is a critical decision to meet the needs of almost all engineering products during their manufacture. The considerations of heating devices and techniques are much different for such industrial applications compared to residential or consumer applications such as hair dryers, hot air popcorn poppers and the like, examples of which are disclosed in U.S. Pat. Nos. 4,350,872; 4,794,255 and 4,149,104. The differences are largely due to the vastly divergent temperature, pressure and airflow requirements. Oven and furnace design for industrial applications must take into consideration heat transfer methods, the temperature uniformity, movement of the product, atmosphere, construction and the heat generation method. Heat processing equipment is usually classified as ovens operating to 1000° C. and as furnaces above this temperature. Batch and continuous designs are the common choices depending on the flexibility and productivity requirements. The source of heat is normally provided by oil, gas or electricity.
Gas heating techniques include natural convection, forced convection and radiation. Natural convection is slow and not very uniform. Forced convection on the other hand is easily controllable and can be directed for odd shapes. Radiant heat transfer at higher temperatures may be faster for some products, but may contribute other problems to the process like non-uniformity and distortion, to mention a few. Forced convection offers advantages over radiant heating for a number of industrial applications. Forced hot convection is also used for fuel cells, automobile test beds and product qualifications.